California will deploy 400 National Guard troops in response to the White House's plan to secure what it insists is a dangerous and porous border with Mexico, but Gov. Jerry Brown warned the troops will not "round up women and children."

The lone Democratic governor of the four southern border states, Brown struck a defiant tone in a letter sent Wednesday to the heads of the Department of Homeland Security and Department of Defense.

"The California National Guard will not be enforcing federal immigration laws," he wrote to DHS Secretary Kirstjen Nielsen and Defense Secretary James Mattis. "Let's be crystal clear on the scope of this mission. This will not be a mission to build a new wall. It will not be a mission to round up women and children or detain people escaping violence and seeking a better life."

His office also clarified that the guards "will not enforce immigration laws or participate in the construction of any new border barrier.”

With the additional troops pledged by Brown, governors from Texas, Arizona, New Mexico and California have committed 1,988 National Guard troops as of Wednesday afternoon. President Trump had told reporters last week he wanted "2,000 to 4,000" troops.

While Brown stated that California would accept federal funding to add approximately 400 members of the National Guard for it's ongoing program to fight transnational crime, it was unclear if the troops would actually be deployed to the southern border.

In his letter, and in a Memorandum of Agreement released by Brown's office, the added National Guard troops from California will be assigned duties they already perform at the border, albeit in smaller numbers, including targeting transnational gangs, human traffickers, illegal firearms, and drug enforcement.

A spokesman for Brown's office said the location of where the troops would be deployed will be "dictated by the needs on the ground."

Brown was the last of the four southern border governors to commit National Guard troops after Trump called for soldiers to patrol the border.

Despite the White House's assertion that the troops would help secure the border, Brown's letter stressed some of the limits National Guard troops would face under the plan, including the fact that existing law prohibits them from arresting or detaining anyone.

Texas Gov. Greg Abbott, whose state plans to deploy more than 1,000 National Guard troops, said in a radio interview Monday troops would handle some of the non-patrol duties of US Border Patrol, freeing agents to patrol the border.

Abbott's commitment of more than 1,000 troops is the largest made by any of the four southwestern state governors who have been contacted by the Department of Homeland Security to coordinate the plan to send National Guard troops to the border.

"Right now, there's 250 on the ground as we speak," Abbott told radio station KTSA Monday. "That's going to be ramped up to more than 1,000 in the state of Texas alone."

Abbott, a Republican, said the plan was to send about 300 troops per week until staffing reached the level wanted by DHS and participating states.

Arizona was the first state to specify the number of soldiers that would be sent to support the Trump administration's operation, with the state's Republican governor, Doug Ducey, stating Friday that 150 troops would be deployed.

On Monday, he upped the number to 338, saying on Twitter that that they will be helping "with any support role responsibilities that they need," and will be stationed in the Tucson and Yuma sectors.

New Mexico's Republican governor, Susana Martinez, also voiced support for the plan. On Monday, her office said about 250 soldiers from New Mexico would serve in the operation, the Associated Press reported.

Brigadier General Tracy Norris of the Texas National Guard on Friday that the first 250 troops would be deployed over the weekend, with notifications going out to the soldiers Saturday so they could notify their families and employers about their orders.

"The Texas National Guard is a force that is trained and ready with firsthand knowledge of this mission, this area, and a longstanding relationship with state and local law enforcement," Norris said.

Norris would not provide further details as to what operations the 250 soldiers would undertake, where they would be sent, or if they would be armed. Those details were still being worked out with DHS and US Customs and Border Enforcement, Norris told reporters.

"We are working with the Department of Homeland Security on what the requirements will be," Norris said. The Texas National Guard already had 100 soldiers on the border conducting "observe and report" duties, she added.

DHS and White House officials have not provided information on what the troops' duties will be. Pentagon regulations prohibit National Guard troops from searching vehicles, or pursuing and arresting anyone, raising questions about how useful their presence will be for border enforcement.

"Working with the border governors, the Department of Homeland Security identified security vulnerabilities that could be addressed by the National Guard," the statement read. "We appreciate the governor's support and are dedicated to working with them to secure the national borders."

The Trump administration's proposal to send National Guard troops to the border comes as detentions at the border hit their lowest numbers since 1971, according to US Customs and Border Protection data. The number of detentions have dropped to 237,176 for the 2018 fiscal year, down from 271,087 during the same period in 2017.

At the same time, however, a caravan of Central American migrants traveling up through Mexico toward the US has become a target of Trump's ire. The hundreds of migrants reached the Mexican city of Puebla aboard buses Friday, and planned to participate in legal seminars to determine who among them will travel on to the United States and who will make asylum claims in Mexico.